In general, the present invention relates to a process for preparing a mercury-barium-calcium-copper-oxide-based superconductor material ("HgBaCaCuO superconductor" or "mercury-based superconductor"). In particular, the present invention relates to process for preparing a HgBaCaCuO superconductor from a precursor mixture containing a lower member of the homologous HgBaCaCuO superconductor series. More particularly, the invention relates to a process for preparing a (Hg.sub.1-x,Re.sub.x)Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8-y superconductor material by annealing a precursor mixture comprising (Hg.sub.1-x,Re.sub.x)Ba.sub.2 Ca.sub.1 Cu.sub.2 O.sub.6-y, a source of calcium and a source of copper, wherein y is a rational number ranging between about negative 1 and postive 1, and x ranges from 0 to about 0.25.
The members of the HgBaCaCuO superconductor series have the nominal formula HgBa.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2-y wherein n is an integer greater than 0 and y is a rational number ranging between about negative 1 and positive 1. There are presently four members of the HgBaCaCuO superconductor series that have been successfully synthesized: HgBa.sub.2 CuO.sub.4-y ("Hg1201"), HgBa.sub.2 CaCu.sub.2 O.sub.6-y ("Hg1212"), HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8-y ("Hg1223"), and HgBa.sub.2 Ca.sub.3 Cu.sub.4 O.sub.10-y ("Hg1234"). These HgBaCaCuO superconductors exhibit excellent superconducting properties, especially for applications above 77K, and have the highest critical temperatures (T.sub.c) of all presently known superconductors. For example, Hg1201, Hg1212 and Hg1223 have critical temperatures of 94K, 128K and 135K, respectively. Hg1212 and Hg1223 have critical current densities on the order of 10.sup.5 A/cm.sup.2 at temperatures up to 100K. At 100K they can have irreversibility fields exceeding 0.5 Tesla.
HgBaCaCuO superconductors are conventionally prepared by encapsulation, high pressure, two-zone annealing or other similar methods. See, for example, Schwartz et al., "HgBaCaCuO Superconductors: Processing, Properties and Potential," Physica B, 216 (1995) 261, which is incorporated herein by reference, which addresses some of the typical methods for preparing HgBaCaCuO superconductors. Most of these methods generally involve the preparation of a HgBaCaCuO superconductor by annealing a precursor mixture comprising oxides of mercury, barium, calcium and copper such as HgO, BaO, CaO, CuO, Ba.sub.2 CaCu.sub.2 O and the like. Other methods use precursor mixtures comprising carbonates, nitrates and even elemental forms of mercury, barium, calcium and copper.
In these conventional processes for the preparation of HgBaCaCuO superconductors, factors affecting the formation of the superconducting phase include the choice and quality of precursor, annealing temperature, annealing time, mercury vapor pressure, and oxygen partial pressure. The precursor materials generally must be kept free from exposure to moisture and carbon dioxide during synthesis of the superconductor material. These synthesis requirements may be lessened by stabilizing the superconducting phase through the incorporation of the atoms of another element, or dopant, into the HgBaCaCuO superconductor structure in place of a portion of the mercury, barium, calcium or copper atoms, a process otherwise known as doping. Doping can occur by intentionally including the element in the precursor mixture used to synthesize the superconductor structure, or indirectly as a result of impurities contained in the precursor materials.
The chemical stability and physical properties of HgBaCaCuO superconductors may be manipulated by using an appropriate dopant or combination of dopants. Among the variety of available dopants, rhenium has shown beneficial effects on the formation and chemical stability of the superconducting phase without reducing the critical temperature. Other dopants such as strontium also can enhance the stability of the superconducting phase. Still other dopants such as lithium, platinum and thallium can enhance the superconducting properties of the HgBaCaCuO superconductor. In fact, Maignan et al., "The Great Ability of Mercury-Based Cuprates to Accommodate Transition Elements" Physica C, 243 (1995) 233-242, incorporated herein by reference, discloses the ability of a number of transition metals to partially replace mercury in HgBaCaCuO superconductors.
A rhenium-doped HgBaCaCuO superconductor requires a higher annealing temperature for the formation of the superconducting phase than does an undoped HgBaCaCuO superconductor. For example, conventional synthesis techniques require an annealing temperature exceeding about 850.degree. C. to prepare a superconductor material having rhenium-doped Hg1223 as the majority phase. Annealing temperatures of less than about 850.degree. C. have been found to be acceptable only if the material is not doped, i.e., if the material is simply Hg1223. Higher annealing temperatures increase the costs associated with the preparation of the HgBaCaCuO superconductor and can complicate the processing of the superconductor into a usable form.
One major requirement for the application of HgBaCaCuO superconductors in technical conductors (such as conductors used for power transmission applications) is the ability to produce the superconductor on a large length scale. The previously discussed preparation methods, however, generally result in superconductor samples having a limited volume. One approach that has been used to produce longer superconductor samples is to encase the HgBaCaCuO superconductor in a substrate or metal sheathing (such as platinum or nickel) and to draw, roll or swage the sheathed material into a usable form prior to annealing. Other approaches used include spray coating, tape casting and other similar methods.
High annealing temperatures and the associated high mercury and oxygen vapor pressures can complicate the preparation of metal-sheathed HgBaCaCuO superconductors. High temperatures and vapor pressures can lead to extensive corrosion and the formation of amalgams between the substrate or sheath and the HgBaCaCuO superconductor. In order to overcome the corrosion and amalgam formation problems, diffusion barriers or buffer materials between the HgBaCaCuO superconductor and the substrate or sheath often are necessary. High annealing temperatures also prevent the use of certain desired sheathing materials. For example, although silver has been successfully used for a metal sheathing for most other high critical temperature superconductors and would be desirable for HgBaCaCuO superconductors, the melting point of silver under typical process conditions is around 800.degree. C. The exact melting point of silver will depend upon the specific mercury and oxygen vapor pressures employed in the process. The melting point, however, is lower than the annealing temperature conventionally employed to synthesize, for example, rhenium-doped Hg1223 superconductor materials. As such, silver typically cannot be used where the annealing temperature is above about 800.degree. C.